The present invention generally relates to wireless communications, and, more particularly, to a wireless communication system in which feedback signals containing channel estimate values are transmitted, and a wireless communication device, a wireless reception device, a wireless communication method, and a channel estimation method that are used in the wireless communication system.
The multi-input multi-output (MIMO) method that has been drawing attention in this field of technology involves the technique of increasing the communication capacity using transmission paths or channels that are formed when multiple transmission antennas and reception antennas are prepared. One application of MIMO is the space division multiplexing (SDM) method by which independent signal streams are transmitted from the transmission antennas through the same frequency band at the same time, so that parallel transmission is realized. Those signals that are transmitted by the above method are combined and received by a receiver. Such a receiver can perform signal separation, using reception signals that are received by the reception antennas. Also, such a receiver can form beams by multiplying transmission weights (weighting factors) in the case where signal streams are transmitted from transmission antennas. Further, in a MIMO channel signal transmitting process, each transmission weight and each reception weight are suitably set so that beams orthogonal to one another can be formed between a transmitter and a receiver. With this fact being taken into account, there is also the eigen beam SDM (ESDM) method by which an independent transmission signal is not transmitted from each transmission antenna, but transmission signal streams are transmitted using the orthogonal beams. Compared with the SDM transmission, the ESDM method is more advantageous in that interference between streams can be reduced. By the ESDM method, a matrix W that is formed with the eigenvectors of a channel matrix is used as a transmission weight on the transmission side, and the conjugate transposition of the product of a channel matrix and an eigenmatrix (HW)H is used as a reception weight on the reception side. A signal stream transmitted from each antenna is then detected. Further, to increase the frequency efficiency, a system that combines the orthogonal frequency division multiplexing method and the ESDM method has also been suggested.
FIG. 1 is a block diagram illustrating a wireless communication device (described below as a transmitter, for ease of explanation) that combines the ESDM method and the OFDM method. The transmitter 100 includes a first serial-to-parallel converter 102, second serial-to-parallel converters 104, the same number of ESDM signal generators 106 as the sub carriers to be used in accordance with the OFDM method, OFDM modulators 108 (including an inverse fast Fourier transformer, a parallel-to-serial converter, and a guard interval inserter) that correspond to transmission antennas, first pilot inserters 110 that correspond to the transmission antennas, and the transmission antennas 112. The transmitter 100 further includes a feedback signal receiver 114, a separator 116, and transmission weight generators 118 that correspond to the respective sub carriers. In general, the relationship among the number Nst of streams to be input to the ESDM signal generators 106, the number N of transmission antennas, and the number M of reception antennas is expressed as 1≦Nst≦min (N,M), where min (N,M) indicates the operation of selecting the smaller one of N and M. For ease of explanation, Nst is equal to N in the following description.
FIG. 2 is a block diagram illustrating a wireless communication device (described below as a receiver, for ease of explanation) that employs both the ESDM method and the OFDM method. The receiver 200 includes reception antennas 202, OFDM demodulators 204 (including a guard interval remover, a serial-to-parallel converter, and a fast Fourier transformer) that are provided for the respective reception antennas, the same number of ESDM signal separators 206 as the sub carriers, the same number of third parallel-to-serial converters 208 as the streams (N), and a fourth parallel-to-serial converter 210. The receiver 200 further includes the same number of pilot channel estimation units 212 as the reception antennas 202, a separator 214, the same number of reception weight generators 216 as the sub carriers, and a feedback unit 218.
As shown in FIG. 1, a data signal (a data symbol) to be transmitted is converted into groups of signals (signal sequences or streams) by the first serial-to-parallel converter 102. Each of the streams after the conversion is further converted into the same number of signals as the sub carriers by the second serial-to-parallel converters 104. The groups of signals after the conversion are provided with transmission weights or weighting factors for each sub carrier component by the ESDM signal generators 106 of the same number as the sub carriers. With the weighting factors, signals that are transmitted from the respective transmission antennas 112 can be distinguished from one another. The weighting factors are determined based on the eigenvalues and the eigenvectors of the corresponding channel matrix. The weighted signal groups are then modulated through inverse fast Fourier transform performed by the same number of OFDM modulators 108 as the transmission antennas 112. The first pilot inserters 110 give pilot signals to the outputs from the corresponding OFDM modulators 108. After guard intervals are inserted, those outputs are transmitted from the transmission antennas 112.
The signals received by the reception antennas 202 shown in FIG. 2 are subjected to fast Fourier transform in the OFDM demodulators 204, and are divided into signals on each sub carrier. The signal on each sub carrier after the OFDM demodulation are further divided into the same number of signal groups as the transmission antennas 112 by the ESDM signal separators 206 using reception weights (weighting factors). The divided signal groups are then converted into the same number of signal streams as the transmission antennas 112 by the third parallel-to-serial converters 208, and are further converted into a single data symbol by the fourth parallel-to-serial converter 210.
Meanwhile, based on the signal groups received by the reception antennas 202, the pilot channel estimation unit 212 estimates the channel impulse response (CIR) values between the transmission antennas 112 and the reception antennas 202. This estimation is carried out by observing how pilot signals vary in the transmission path between the transmission end and the reception end. In this example, the channel impulse values of the cth sub carrier component between the mth reception antenna and the nth transmission antenna is expressed as hcmn. In the following description, symbols with the subscript “c” is an index of sub carrier. The matrix Hc that has the respective channel impulse response values hcmn as matrix elements is referred to as a channel matrix, and is expressed as follows:
                              H          c                =                  [                                                                      h                  c11                                                                              h                  c12                                                            …                                                              h                                      c1                    ⁢                    N                                                                                                                        h                  c21                                                                              h                  c22                                                            …                                                              h                                      c2                    ⁢                    N                                                                                                      ⋮                                            ⋮                                            ⋰                                            ⋮                                                                                      h                                      c                    ⁢                    M                    ⁢                    1                                                                                                h                                      c                    ⁢                    M2                                                                              …                                                              h                                      c                    ⁢                    MN                                                                                ]                                    (                  Equation          ⁢                                          ⁢          1                )            
where N indicates the number of transmission antennas 112, and M indicates the number of reception antennas 202. The information as to the channel impulse response values is input to the reception weight generators 216 that are prepared for the respective sub carriers. Each of the reception weight generators 216 determines the eigenvalue λcn and the eigenvector wcn (1≦n≦N) of the matrix represented by HcHHc, and supplies the amount representing (Hcwcn) to the corresponding ESDM signal separator 206. Here, the operator represented by the superscript “H” is the conjugate transpose. The eigenvector wcn is the vector that has the same number of components as the transmission antennas 112.
Meanwhile, the cth sub carrier component rc contained in reception signals is expressed as:rc=rc1+ . . . +rcN 
where rcn can be expressed as:rcn=Hcwcnscn 
Alternatively, rcn may be expressed as:rcn=(rc1n . . . rcMn)T 
where T indicates the transpose. Also, scn indicates the signal component transmitted on the cth sub carrier among the transmitted signals. Accordingly, at each of the ESDM signal separators 206, the transmitted signal can be determined by multiplying the reception signal rcn=Hcwcnscn by (Hcwcn)H. This is because the relationship between the eigenvalue λcn and the eigenvector wcn can be expressed as:(Hcwcn)H(Hcwcn)=λcn (Hcwcn)H(Hcwcn)=0(n≠j)
Meanwhile, the channel impulse response values or the information as to the channel matrix estimated by the pilot channel estimation units 212 are fed back to the transmitter 100 through the feedback unit 218.
The transmitter 100 shown in FIG. 1 receives the information, which is fed back from the receiver 200, through the feedback signal receiver 114, and divides the information into sets of information corresponding to the sub carriers at the separator 116. The divided information is the information as to the channel value of each sub carrier, and is supplied to the transmission weight generators 118 corresponding to the sub carriers. The transmission weight generators 118 calculate the weighting factors wcn of the respective sub carriers. The transmitter 100 uses the weighting factors for the next transmission, instead of the previously used weighting factors.
An example of the ESDM wireless communication technique is disclosed in Miyashita, et al., Shingakugiho, RCS 2002-53.
The feedback signals are transmitted from the receiver 200 to the transmitter 100, because the channel estimate value h of the forward link that is directed from the transmitter 100 to the receiver 200 is normally different from the channel estimate value h′ of the backward link that is directed from the receiver 200 to the transmitter 100. In short, the frequency division duplexing (FDD) method is supposed to be employed here. However, it is not necessary to transmit feedback signals in accordance with the time division duplexing (TDD) method in which the channel estimate values are equal to each other of the forward link and the backward link.
FIG. 3 is a block diagram showing the feedback unit 218 of the receiver 200 of FIG. 2. As shown in FIG. 3, the channel information representing the channel estimate value h is quantized to a quantization level suitable for feedback signals by a feedback quantizing unit. To reduce the information amount of the feedback signals, the small number of quantization levels should be used. The quantized binary signals are then subjected to error correction encoding by an encoder, and are output to an interleaver. The signals rearranged by the interleaver are mapped to suitable symbols by a symbol mapping unit. The mapped signals are then multiplexed with pilot signals. The multiplexed signals are then converted into parallel signals by the serial-to-parallel converter (S/P) in an OFDM modulator indicated by a broken-line frame in FIG. 3. The converted signals are further subjected to inverse fast Fourier transform in the OFDM modulator. In this manner, feedback signals containing channel information are generated and transmitted to the transmitter 100. In the case of employing a single-carrier method, instead of the OFDM method, the serial-to-parallel converter (S/P) and the IFFT unit indicated in the broken-line frame are omitted.
FIG. 4 is a block diagram showing the feedback signal receiver 114 of the transmitter 100 of FIG. 1. As shown in FIG. 4, the received feedback signals are subjected to fast Fourier transform in an OFDM demodulator indicated by a broken-line frame. The feedback signals are thus converted into signal groups of the frequency regions, and are further converted into series signals by a parallel-to-serial converter (P/S). Based on those signals, channel estimation and channel compensation are carried out. Based on the signals after the channel compensation, symbol decision is carried out at a demapping unit. The signals after the decision are rearranged in a predetermined order by a deinterleaver. The rearranged signals are subjected to error correction decoding by a decoder. The channel estimate value h, which is reproduced based on the bit stream as the result of the decoding is output from a channel reproducing unit. In the case of employing a single-carrier method, instead of the OFDM method, the FFT unit and the parallel-to-serial converter (P/S) indicated in the broken-line frame in FIG. 4 are omitted.
As described above, a large number of signals and a large amount of signal processing are required for transmission and reception of feedback signals. By the ESDM transmission technique of the FDD method, it is necessary to feed all the channel information back to the transmitter from the receiver. Accordingly, the amount of feedback becomes very large. As a result, the resources for transmitting signals such as data signals other than feedback signals might be insufficient. This problem becomes more severe when wireless communication is performed in wide bands, as even more feedback signals are transmitted.